1. Field of the Invention
The present invention relates to a radiation image detector which includes the following layers stacked on top of another: a charge generation layer that generates charges by receiving radiation; and a detection layer in which multitudes of pixels, each having a TFT switch, are disposed two dimensionally.
2. Description of the Related Art
Recently, flat panel detectors (FPDs) have been put into practical use. FPD includes an X-ray sensitive layer on a TFT active matrix array and is capable of directly converting X-ray information to digital data. It has advantages over conventional imaging plates in that it allows instantaneous image verification and checking for motion images, and is spreading rapidly.
First, the configuration of a conventional radiation image detector will be described with reference to FIG. 10.
In the conventional radiation image detector, a semiconductor layer 20 which is conductive for electromagnetic waves is formed on an active matrix substrate 10 having collection electrodes 8 disposed in an array, and an upper electrode 22 is formed on the semiconductor layer 20, as illustrated in FIG. 10. The upper electrode 22 is connected to a high voltage power source 24. The semiconductor film 20 is a selenium-based amorphous a-Se film with a thickness of 100 to 1000 μm, and generates charges inside of the film when exposed to X-rays. A TFT switch 3 and a storage capacitor 4 are provided adjacent to each of collection electrodes 8 disposed on the active matrix substrate 10 in an array. The drain electrode 7 of the TFT switch 3 is connected to one of the electrodes of the storage capacitor 4. The other electrode of the storage capacitor 4 is connected to a storage capacitor wire 12. A scanning line 1 is connected to the gate electrode 2 of the TFT switch 3, and a data line 5 is connected to the source electrode 6. An amplifier 23 is connected to the end of the data line 5.
The operational principle of the conventional radiation image detector will be described next.
When X-rays are irradiated from above in FIG. 10, the semiconductor film 20 generates charges inside thereof. Holes of the charges generated in the semiconductor film 20 are collected to each collection electrode 8 due to a bias between the upper electrode 22 and collection electrode 8, and stored in the storage capacitor 4 electrically connected to the collection electrode 8. The semiconductor film 20 generates different amounts of charges depending on the X-ray dosage, so that an amount of charges depending on image information represented by the X-rays is stored in the storage capacitor 4 of each pixel. Thereafter, a signal for switching ON each TFT switch 3 is sequentially applied through each scanning line 1, and charges stored in each storage capacitor 4 are read out through each data line 5. Then, the amount of charges of each pixel is detected by each amplifier 23, thereby the image information is read out.
Here, in the currently available radiation image detectors, the common pixel size is around 100 to 300 μm square. In order to improve image quality of X-ray images, a finer pixel resolution is demanded.
The pixel size, however, is practically limited to around 100 μm square due to restrictions arising from the TFT array structure. The reason for this will be described with reference to a layout chart of the conventional radiation image detector shown in FIG. 11. In the conventional TFT layout structure, a storage capacitor wire 12 is disposed between the scanning lines 1 along them in order to form the storage capacitor 4. The storage capacitor wire is formed of the same layer metal as the scanning line 1 in order to reduce production costs.
Here, for example, assuming to realize a radiation image detector with a pixel pitch of 50 μm using the aforementioned pixel layout. If the width of the scanning line 1 and storage capacitor wire 12 is 12 μm, then the distance available between the scanning line 1 and storage capacitor wire 12 is only 13 μm. As described above, the scanning line 1 and storage capacitor wire 12 are on the same layer, so that the decrease in the distance between them leads to decrease in the yield rate due to increase in the interline leaks. Formation of the scanning lines and storage capacitor wires in different layers additionally requires a wiring layer and an insulation layer. This is not practical because of a significant increase in the production cost.
Here, a pixel layout without requiring the storage capacitor wiring is conceivable in which each of the storage capacitors 4 is formed by overlapping a storage capacitor electrode 9 connected to the drain 7 of each of the TFT switches 3 with a scanning line 1 connected to a TFT switch adjacent to each of the TFT switches 3 through an insulation film as illustrated in FIG. 3. The aforementioned pixel layout allows the spacing of the scanning lines to be increased. For example, when the pixel pitch is 50 μm and the width of the scanning line is 12 μm, a distance of 38 μm is ensured between the scanning lines, thereby the interline leakage may be prevented.
In the mean time, the number of pixels of such TFT array type radiation image detector has increased along with higher resolution and larger size. As a result, the electrical noise has also increased. Consequently, a vertical division method is adopted in a large radiation image detector, in which data lines are divided into upper and lower halves at a center portion of the TFT switch array, and amplifier ICs are connected to the TFT switch array at the upper and lower sides.
If the vertical division method is adopted in the pixel layout illustrated in FIG. 3, however, the storage capacitors 4 for the pixels at the division boundary section are unable to be formed as clear from the equivalent circuit shown in FIG. 12 or other drawings and the layout shown in FIG. 13.
In view of the circumstances described above, it is an object of the present invention to provide a vertical division radiation image detector like that as described above which is capable of forming storage capacitors for the pixels at the division boundary section and making a step difference in the image at the division boundary section less noticeable.